When a cellulosic starting material is treated in the presence of a catalytic amount of 2,2,6,6-tetramethyl-1-piperidin-N-oxyradical (hereinafter “TEMPO”) and sodium hypochlorite which is an inexpensive oxidizing agent, carboxyl groups can be introduced efficiently onto the cellulose microfibril surface. It is known that the cellulosic starting material onto which carboxyl groups have been introduced can be used to prepare a high-viscosity and transparent aqueous dispersion of cellulose nanofibers by treating the cellulosic starting material through a simple mechanical process in water using a mixer or the like (Non-patent Document 1).
Cellulose nanofibers are a novel water-dispersible biodegradable material. The properties of cellulose nanofibers can be modified using, as base points, carboxyl groups introduced onto the surface of the cellulose nanofibers. Further, since the cellulose nanofibers obtained by the method mentioned above are in the form of a dispersion liquid, they can be blended with various water-soluble polymers, can form a composite with an organic or inorganic pigment and can also be formed into a sheet or fiber. These characteristics of cellulose nanofibers are expected to be of help in using cellulose nanofibers in highly functional packaging materials, transparent organic substrate materials, highly functional fibers, separation membranes, regenerative biomaterials and the like and applying cellulose nanofibers to the development of novel and highly functional products.
In the production of cellulose nanofibers, kraft pulp (KP), sulfite pulp (SP), natural cellulose and the like have been so far proposed as cellulosic starting materials (Patent Document 1, Patent Document 2). Among them, sulfite pulp (SP) or kraft pulp (KP) for papermaking, which is highly reactive in oxidation reaction, inexpensive and easy to acquire as a starting material, is generally used. Meanwhile, no examples using pulp obtained by hydrolysis treatment and subsequent kraft cooking (DKP) as a cellulosic starting material have been reported.
Pulp obtained by hydrolysis treatment and subsequent kraft cooking (DKP) is one of dissolving pulps (DPs). Dissolving pulp (DP) is chemically highly purified pulp with high cellulose purity (Non-patent Document 2) and used chiefly as a starting material for viscose rayon, cellophane, fibers such as acetate and cupra, carboxymethyl cellulose (CMC) and the like, rather than for papermaking (Non-patent Document 3).